THE HYDRATION OF MAGNESIUM OXIDE WITH DIFFERENT REACTIVITIES BY WATER AND MAGNESIUM ACETATE by MATHIBELA ELIAS APHANE submitted in fulfilment of the requirements for the degree of MASTER OF SCIENCE in the subject CHEMISTRY at the UNIVERSITY OF SOUTH AFRICA SUPERVISOR: DR E M VAN DER MERWE JOINT SUPERVISOR: PROF C A STRYDOM MARCH 2007 DECLARATION BY CANDIDATE I hereby declare that THE HYDRATION OF MAGNESIUM OXIDE WITH DIFFERENT REACTIVITIES BY WATER AND MAGNESIUM ACETATE is my own original work and has not previously been submitted to any other institution of higher learning. I further declare that all the sources that I have used or quoted have been indicated and acknowledged by means of a comprehensive list of references. ................................................ M. E. APHANE i ACKNOWLEDGEMENTS I would like to express my deepest gratitude towards the following people, institutions and companies that contributed in making this study a success: • My supervisor, Dr Liezel van der Merwe for her patience, guidance and comments during the study, advice, support and encouragement. I would also like to thank her for inviting me to join her in this research and for having trust in me. • My co-supervisor, Prof. C. A. Strydom for her appreciation in my work and for her helpful comments. • University of South Africa and NRF for financial support in my study. • Magnesium Compounds Consortium for providing the MgO sample. • Department of Chemistry for their facilities. • Ms Maggie Loubser of University of Pretoria for doing the XRF and XRD analysis and interpretation of the results. • I am grateful to Ms E. Ten Krooden and Mrs Tryphina Moeketsi for their assistance in finding the library books and journals. • University of Pretoria for using their facilities. • I am grateful to all Chemistry staff at UNISA. • Thanks to the chairperson of chemistry department, Prof. M. J. Mphahlele for all the support in the postgraduate assistantship program. ii DEDICATION This study was dedicated to the following people: • My late grandfather: Mathibela Elias Aphane the senior • My grandmother: Monere Stephina Aphane • My wonderful mother: Majalla Elizabeth Aphane • My only sister: Rakgadi Aphane • My late aunt: Leah Masadi Aphane, may your soul rest in peace • My son: Tumelo • My fiancée: Minkie iii ABSTRACT The use of magnesium hydroxide (Mg(OH)2) as a flame retardant and smoke- suppressor in polymeric materials has been of great interest recently. Because it contains no halogens or heavy metals, it is more environmentally friendly than the flame retardants based on antimony metals or halogenated compounds. Mg(OH)2 can be produced by the hydration of magnesium oxide (MgO), which is usually produced industrially from the calcination of the mineral magnesite (MgCO3). The thermal treatment of the calcination process dramatically affects the reactivity of the MgO formed. Reactivity of MgO refers to the extent and the rate of hydration thereof to Mg(OH)2. The aim of this study was to investigate the effect of calcination time and temperature on the reactivity of MgO, by studying the extent of its hydration to Mg(OH)2, using water and magnesium acetate as hydrating agents. A thermogravimetric analysis (TGA) method was used to determine the degree of hydration of MgO to Mg(OH)2. The reactivity of MgO was determined by BET (Brunauer, Emmett and Teller) surface area analysis and a citric acid reactivity method. Other techniques used included XRD, XRF and particle size analysis by milling and sieving. The product obtained from the hydration of MgO in magnesium acetate solutions contains mainly Mg(OH)2, but also some unreacted magnesium acetate. Magnesium acetate decomposition reaction takes place in the same temperature range as magnesium hydroxide, which complicates the quantitative TG analysis of the hydrated samples. As a result, a thermogravimetric method was developed to quantitatively determine the amounts of Mg(OH)2 and Mg(CH3COO)2 in a mixture thereof. iv The extent to which different experimental parameters (concentration of magnesium acetate, solid to liquid ratio and hydration time) influence the degree of hydration of MgO were evaluated using magnesium acetate as a hydrating agent. Magnesium acetate was found to enhance the degree of MgO hydration when compared to water. By increasing the hydration time, an increase in the percentage of Mg(OH)2 formed was observed. In order to study the effect of calcining time and temperature on the hydration of the MgO, the MgO samples were then calcined at different time periods and at different temperatures. The results have shown that the calcination temperature is the main variable affecting the surface area and reactivity of MgO. Lastly, an attempt was made to investigate the time for maximum hydration of MgO o calcined at 650, 1000 and 1200 C. From the amounts of Mg(OH)2 obtained in magnesium acetate, it seems that the same maximum degree of hydration is obtained after different hydration times. A levelling effect that was independent of the calcination temperature of MgO was obtained for the hydrations performed in magnesium acetate. Although there was an increase in the percentage of Mg(OH)2 obtained from hydration of MgO in water, the levelling effect observed in magnesium acetate was not observed in water as a hydrating agent, and it seemed that the extent of MgO hydration in water was still increasing. The results obtained in this study demonstrate that the calcination temperature can affect the reactivity of MgO considerably, and that by increasing the hydration time, the degree of hydration of MgO to Mg(OH)2 is enhanced dramatically. v CONTENTS DECLARATION BY CANDIDATE i ACKNOWLEDGEMENTS ii DEDICATION iii ABSTRACT iv CONTENTS vi ABBREVIATIONS USED xii NOMENCLATURE xiii LIST OF MINERALS xv CHAPTER 1 INTRODUCTION 1.1 Aims of the study on the hydration of MgO 1 1.2 Physical properties of Mg(OH)2, MgO and MgCO3 3 1.2.1 Physical properties of magnesium hydroxide 3 1.2.2 Physical properties of magnesium oxide 3 1.2.3 Physical properties and occurrence of magnesium carbonate 3 1.3 Production of magnesium oxide 4 1.4 Grades of magnesium oxide 6 1.5 The reactivity of magnesium oxide 8 1.6 Applications and uses of magnesium oxide 10 1.6.1 Agricultural uses 10 1.6.2 Environmental uses 10 1.6.3 Refractory applications 10 1.6.4 Other applications 11 1.7 Production of magnesium hydroxide 11 1.7.1 Magnesium hydroxide from MgO 11 1.7.2 Magnesium hydroxide from brines or sea water 12 vi 1.8 Applications and uses of magnesium hydroxide 13 1.8.1 Application of Mg(OH)2 in industrial water treatment 13 1.8.2 Mg(OH)2 as a flame-retardant filler 14 1.9 Previous studies related to this study 17 1.9.1 Hydration of MgO to Mg(OH)2 in water 17 1.9.2 Hydration of MgO to Mg(OH)2 in magnesium acetate 20 1.9.3 Mechanism of MgO hydration 21 CHAPTER 2 Experimental techniques and methods applied in this study 2.1 Thermal Analysis (TA) 25 2.1.1 Introduction 25 2.1.2 The main thermal analysis techniques 26 2.1.3 Thermal events 28 2.1.4 Thermal analysis equipment 29 2.2 Thermogravimetric analysis (TGA) 31 2.2.1 Thermogravimetric instrument 31 2.2.2 The electronic microbalance 33 2.2.3 Heating the sample 34 2.2.4 The atmosphere 34 2.2.5 The sample and crucibles (sample pans) 36 2.2.6 Measurements of temperature and calibration 37 2.2.7 Precautions when performing TG 38 2.2.8 Typical TG curves 39 2.2.9 Applications of TG 42 2.3 BET Surface Area Analysis 43 2.3.1 Introduction 43 2.3.2 Adsorption 43 2.3.3 Determination of surface area: BET theory 44 2.4 Citric acid reactivity test 47 2.5 X-ray Fluorescence analysis (XRF) 49 2.5.1 Introduction 49 vii 2.5.2 Generation of X-rays 50 2.5.3 Characteristic radiation 52 2.5.4 Instrumentation: The X-ray Spectrometer 53 2.5.5 XRF qualitative analysis 59 2.5.6 XRF quantitative analysis 59 2.5.7 Sample preparation for XRF analysis 60 2.6 X-ray Diffraction analysis (XRD) 62 2.6.1 Introduction 62 2.6.2 Sources and Detectors for X-radiation 63 2.6.3 Principles of X-ray Diffraction 64 2.6.4 Instrumentation: The powder diffractometers 64 2.6.5 Qualitative XRD analysis 67 2.6.6 Sample preparation for XRD analyses 68 CHAPTER 3 Thermogravimetric analysis of known mixtures of Mg(OH)2 and Mg(CH3COO)2 3.1 Introduction 70 3.2 Experimental 71 3.2.1 Samples 71 3.2.2 Sample preparation and experimental procedure 71 3.3 Instrumental analysis 71 3.3.1 TG analysis 71 3.4 Results and Discussion 72 3.5 Conclusion 80 CHAPTER 4 The effect of different experimental parameters on the hydration of MgO 4.1 Introduction 81 4.2 Experimental 82 4.2.1 Samples 82 viii 4.2.2 Sample preparation 82 4.2.3 Experimental parameters and procedure 82 4.2.4 Citric acid test 83 4.3 Instrumental analysis 83 4.3.1 TG analysis 83 4.3.2 XRD analysis 84 4.3.3 XRF analysis 84 4.3.4 BET surface area analysis 84 4.4 Results and Discussion 85 4.4.1 XRD, XRF and surface area analyses of the raw and dried magnesite 85 4.4.2 TG analysis of raw magnesite and dried magnesite 86 4.4.3 Citric acid reactivity test performed on MgO samples 87 4.4.4 Effect of varying the Mg(CH3COO)2 concentration between 0 and 0.2 M 87 4.4.5 Surface area analyses 92 4.5 Conclusion 93 CHAPTER 5 The effect of calcination time on the hydration of MgO 5.1 Introduction 94 5.2 Experimental 96 5.2.1 Samples 96 5.2.2 Sample preparation 96 5.2.3 Experimental parameters and procedure 96 5.2.4 Citric acid test 97 5.3 Instrumental analysis 97 5.3.1 TG analysis 97 5.3.2 BET surface area analysis 97 5.4 Results and Discussion 98 5.5 Conclusion 101 ix CHAPTER 6 The effect of MgO reactivity